1. Field of the Invention
The present invention relates to a communication method and a communication system for performing communication by using a signal multiplexed by at least one of wavelength-division multiplexing and time-division multiplexing.
2. Related Background Art
Recently, a system loading a large quantity of information such as motion picture information in a computer network has been studied. For example, a wavelength multiplexing communication system such as a WDM (wavelength-division multiplexing) composite type optical LAN (local area network) described in OQE91-126 has been proposed. In this system, a plurality of wavelengths .lambda..sub.a and .lambda..sub.1 to .lambda..sub.n are used. Communication of low-speed data and wavelength control information is performed through an existing LAN by using the wavelength .lambda..sub.a, whereas the wavelengths .lambda..sub.1 to .lambda..sub.n are used to perform communication services which are independent of speed (i.e., which may include services of various speeds) by wavelength multiplexing transmission. Signals of these wavelengths .lambda..sub.a and .lambda..sub.1 to .lambda..sub.n are accommodated in one loop type transmission path.
As a wavelength multiplexing transmission scheme, a demand-assign wavelength-division multiplexing access (DA-WDMA) scheme is employed. In this scheme, when high-speed communication or communication with a long holding time is requested between some nodes, a specific wavelength is assigned to the communication between the nodes, and a channel is formed by the wavelength to perform the communication.
The wavelength assignment method will be described in detail below with reference to FIGS. 1 and 2. FIG. 1 is a block diagram showing the function of each node in the system described above. Each node includes a communication control circuit 16 for an existing LAN, a communication control circuit 17 of the DA-WDMA scheme, an interface circuit 18, a demultiplexer 10, a multiplexer 11, and an optical transmission path 20. The interface circuit 18 connects a signal from a terminal (not shown) to the communication control circuit 16 for the existing LAN or the communication control circuit 17 of the DA-WDMA scheme depending on the type of signal (e.g., a signal with a small quantity of information is connected to the communication control circuit 16 for the existing LAN, and a signal with a large quantity of information is connected to the communication control circuit 17 of the DA-WDMA scheme). FIG. 2 shows the network arrangement of the system. A control node 201 manages wavelengths used for communication by the respective nodes. Nodes 202 to 205 serve to perform communication between terminals.
A method of transmitting high-speed data from the node 203 to the node 205 will be described first. When the node 203 generates a request to transmit high-speed data such as video data, the interface circuit 18 supplies the transmission request and a reception node information signal to the communication control circuit 16 for the existing LAN. The communication control circuit 16 for the existing LAN converts the information into an optical signal having the wavelength .lambda..sub.a and transmits it by an existing communication scheme such as a token passing scheme, a TDMA (time-division multiplexing access) scheme, or a slotted loop scheme. This optical signal is accommodated in one optical transmission path by the multiplexer 11 and is output to the optical transmission path 20. The optical signal then reaches the control node 201 through the nodes 204 and 205. The control node 201 has a wavelength table and manages the use of the respective wavelengths by specific nodes. First, the control node 201 checks the state of the node 205. If the node 205 is receiving a signal, the node 203 is set in a standby state. If the node 205 is not receiving a signal, the control node 201 assigns a transmission wavelength according to the wavelength table and informs the wavelength information to the node 205 to cause it to prepare for reception. The control node 201 then informs the wavelength information to the node 203 to cause it to start transmission. A signal from the control node 201 is transmitted through the existing LAN by using the wavelength .lambda..sub.a. The signal is input to the nodes 205 and 203. In each node, the signal is separated from signals having other wavelengths by the demultiplexer 10. The signal is then input to the communication control circuit 16 for the existing LAN. The communication control circuit 16 for the existing LAN informs the assigned wavelength to the communication control circuit 17 of the DA-WDMA scheme. In the node 203, the wavelength of an optical transmitter in the communication control circuit 17 of the DA-WDMA scheme is set to the assigned wavelength to transmit the video signal. In the node 205, the wavelength of the filter of an optical receiver in the communication control circuit 17 of the DA-WDMA scheme is set to the assigned wavelength to receive the video signal from the node 203. When the transmission from the node 203 is completed, the node 203 informs the end of transmission to the control node 201. The control node 201 updates the wavelength table. The node 205 cancels the setting of the filter. With this operation, the communication is completed.
Similarly, wavelengths are assigned among other nodes by the control node 201 to perform communication. In this manner, assignment of wavelengths is managed by the control node 201 alone.
In the conventional system described above, since the control node manages all the wavelengths, the control node is required to have high reliability, resulting in an increase in the cost of the control node. In addition, since communication is performed by instructions from the control node, the communication procedure is complicated.